1. Field of the Invention
The present invention relates to a novel propylene block copolymer and a process for production thereof, and more specifically relates to a novel propylene block copolymer with excellent flexibility or impact resistance, good antiblocking property without sticky feeling, less gel generation in molding, low coefficient of linear expansion, and good polymer powder form, obtained by using a metallocene catalyst supported on a carrier, and a process for production thereof stably and efficiently.
2. Description of the Prior Art
Crystalline polypropylene is widely used in various molding fields because it is excellent in mechanical property, chemical resistance or the like. However, when a propylene homopolymer, or a random copolymer with a small amount of an α-olefin is used as the crystalline polypropylene, stiffness is high, but impact resistance is insufficient. Thus improvement of impact resistance has been carried out by a method for adding elastomer such as ethylene-propylene rubber or the like to a propylene homopolymer or a random copolymer; or a method in which after homopolymerization of propylene or random copolymerization with a small amount of an α-olefin, propylene is subsequently copolymerized with ethylene or an α-olefin to produce a so-called block copolymer. Furthermore, it is known that by increasing amount of a rubber part of this block copolymer, a propylene block copolymer with improved flexibility or impact resistance can be obtained.
In addition, as a different problem from this, a low molecular weight component (an oligomer or the like) always presents, due to property of the catalyst, in a propylene block copolymer obtained by polymerization in the presence of a conventional Ziegler-Natta type catalyst. Recently, in particular, there is a tendency to more improve moldability of the resultant propylene block copolymer by increasing fluidity. However, it is known that too much increase in fluidity as for the rubber component results in increase in also generation ratio of a low molecular weight component therewith, and this low molecular weight component not only causes various problems such as generation of smoke, malodor or the like in molding, but also gives bad effect on odor or taste even after molding, or deterioration of antiblocking property caused by sticking or the like, and thus raised a problem of inhibiting stable production caused by deterioration of polymer powder form. On the other hand, it is also known that increase in difference between average molecular weight of crystalline polypropylene and that of a rubber part generates a problem of providing more gel in molding or making coefficient of linear expansion high.
On the other hand, it is known that an isotactic polypropylene can be obtained by polymerization of propylene using a metallocene type catalyst different from a conventional Ziegler-Natta type catalyst. In addition, it is also known that a so-called a block copolymer is produced by homopolymerization of propylene by using a similar catalyst and subsequently by copolymerization of ethylene and propylene (for example, see Patent Document 1 to 5). In addition, a process is disclosed for producing a block copolymer, wherein a copolymer of ethylene and propylene is produced in a former step, and homopolymerization of propylene is carried out in a latter step (for example, see Patent Document 6).
Furthermore, a propylene-ethylene block copolymer having good stiffness and impact resistance (for example, see Patent Document 7 to 10) is disclosed. Still further, a propylene resin composition excellent in flexibility or transparency, and that without sticky feeling (for example, see Patent Document 11 and 12) is disclosed.
However, according to the above proposals, though stiffness and impact resistance are further improved, amount of a copolymer of propylene and ethylene or an α-olefin, or a comonomer composition of the copolymer, which furnishes flexibility or impact resistance, has not yet reached a satisfactory level, and thus improvement is desired, and also, development of a process for production to stably produce a propylene block copolymer having such a satisfactory level has been desired. As far as the present inventors know, there have been no examples of producing such a propylene copolymer by using a metallocene catalyst supported on a carrier, and by carrying out the latter step for polymerization of a rubber part, in gas phase polymerization.
Patent Document 1: JP-A-4-337308
Patent Document 2: JP-A-6-287257
Patent Document 3: JP-A-5-202152
Patent Document 4: JP-A-6-206921
Patent Document 5: JP-A-10-219047
Patent Document 6: JP-A-8-27237
Patent Document 7: JP-A-11-228648
Patent Document 8: JP-A-11-240929
Patent Document 9: JP-A-11-349649
Patent Document 10: JP-A-11-349650
Patent Document 11: JP-A-2000-239462
Patent Document 12: JP-A-2001-64335